Tail-Gas Preheater

Weak Acid. Stainless steels (SS) have exceUent corrosion resistance to weak nitric acid and are the primary materials of constmction for a weak acid process. Low carbon stainless steels are preferred because of their resistance to corrosion at weld points. However, higher grade materials of constmction are required for certain sections of the weak acid process. These are limited to high temperature areas around the gau2e (ca 900°G) and to places in which contact with hot Hquid nitric acid is likely to be experienced (the cooler condenser and tail gas preheater). 

Tail-gas preheater Cool reaction gases and preheat tail gas. 

Tail-gas warmer Provide first stage of tail gas preheat. 

Note (a) The waste-heat boiler, tail-gas preheater, cooler/condenser, and oxidation unit form a sub-system of the process. 

Tail-gas Preheater A shell and tube-type exchanger. It takes reaction gases leaving the platinum filter at about 315°C and 1020 kPa, and subsequently reduces their temperature to 185°C. The... 

Tail-gas preheater heat-transfer area = 89m2. 

Application -Tail-gas preheater Heat-exchange area = 89 m ... 

The tail-gas preheater is the second stage of preheat for the tail gas. It uses sensible heat and reaction heat from the process reaction gases to preheat the tail gas (ultimately for expansion). 

The mass flowrate of tail gas is known from the overall mass balance, but the tail-gas preheater balance can only be performed if the inlet temperature of the tail gas is known, This must be calculated from the specified design conditions for the absorber, and from the heat exchange at the primary tail-gas warmer. Tail gas leaves the absorber at 10°C (design specification) and it passes through the primary tail-gas warmer where it exchanges heat with the secondary air (for the product-acid bleaching column). The air enters at 232°C and leaves at 80°C. Assuming the tail gas has approximately the same heat capacity as air (a realistic assumption), then ... 

Therefore, tail gas enters the tail-gas preheater and is expected to increase its temperature to 235°C (from calculations for the reactor). The average heat capacity for the tail gas in the temperature range of 45°C to 235°C is 1.05 kJ/(kg K). 

High chromium (20% to 27%) stainless steel is used in the cooler condenser and the tail gas preheater. Compared to low carbon stainless steel, it has better corrosion resistance to nitric acid at elevated temperatures so the extra cost is justified by the longer life104. 

Zirconium is used in nitric acid service for cooler condensers, tail gas preheaters and reboilers. It rivals tantalum in its corrosion resistance to nitric acid at all concentrations up to the boiling point. Its resistance extends up to 230°C and 65 wt %. However it is susceptible to stress corrosion cracking, which can be prevented by avoiding high, sustained tensile stresses104. 

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